The invention relates to a method and apparatus for sizing grains smaller than 300.mu.. According to the method, the grains suspended in a carrier medium are led to the surface or ducts of a rotary element, meanwhile a sizing medium is introduced on the level of the rotary element in radial direction towards the axis of rotation, the coarse fraction falling down at the flange of the rotary element, and the fine fraction carried off from the axis of rotation, are collected separately. The apparatus is provided with a house and an impeller deflecting elements arranged thereon wherein the house is provided with inlet tubes for the admission of a suspension and a sizing medium as well as with fine and coarse fraction outlet tubes and/or nozzles.
It is well-known that hydrocyclones are generally used for the sizing of fine grains. Hydrocyclones have been used for a long time, however their fundamental drawback is that the separation accomplished with them is not sufficiently sharp. The further-developments of these apparatuses (see for example the West-German Pat. Nos. 2,536,350 or 2,942,099) ensure only very limited result.
Another well-known group of the sizers is represented by the so-called hydraulic separators, functioning in liquid flowing upward within a large tube. The separation is based on the principle that the grains of higher falling velocity than the velocity of the medium fall to the bottom of the tube, while the fine grains move off with the medium on the top.
Their drawback is that the grains fall slowly in the gravitational field, hence the separation only of large sizes is possible, and their output is low even in case of a large diameter. Upon increasing the diameter, the sharpness of separation quickly deteriorates, because a perfectly laminar flow cannot be ensured in the expanding cross sections.
The more up-to-date apparatuses functioning with gaseous medium are the so-called dispersive air separators (see for example the West-German Pat. No. 25 56 383). Here, a spray disc spreading the material and a fan blowing the air are arranged in the upper part of the assorting space. Sharpness of the sizing is inferior to the liquids, since the rotary part induces heavy turbulences disturbing the sizing. These apparatuses, owing to their poor sizing effect, are used only for intermediate, temporary tasks in the preparation technologies.
Known are furthermore the so-called spiral or zig-zag air separators. Such apparatus is disclosed for example in the West-German Patent 2 529 745. These apparatuses consist of a rotary impeller arranged in a stationary house and the spiral or zig-zag paths are formed between the ribs on the impeller's plate. The material to be separated is guided by a carrier medium to the flange of the rotary impeller, where the large grains fall down, while the smaller ones are entrained by the axially injected or induced sizing medium, and they are leaving the apparatus on such spiral path along which an identical discharge force is applied to the grains both in the eddy field or in the rotary ducts.
The sizing of these apparatuses is relatively better than that of those mentioned in the practice, however, they do not ensure a theoretically perfect path curve, nor a perfect sizing. In the zig-zag type apparatuses the gap size does not change according to the requirement that the discharge force should remain constant in the gap duct. Moreover the sizing is imperfect, because, though the fine fraction does not contain coarse grains, the fine grains not getting into the ducts of the impeller, pass with the coarse grains into the coarse fraction.
Such aerodynamic sizer also exists (Hungarian patent application No. 2429/85), which ensures a constant lifting power for the grains to be sized in a theoretically perfect flow tube. This can be accomplished in the apparatus consisting of a house, inlet stub, fine fraction outlet stub and coarse fraction outlet stub, as well as blade crowns connecting the inlet stub to an annular inlet duct. The outlet stubs are arranged vertically and coaxially, and the house is provided with an inlet blade crown and an outlet blade crown. Furthermore the separating or sizing chamber is formed with a rotational hyperboloid mantle between the inlet blade crown and the outlet blade crown.
Although this apparatus provides very good separation, at a given size (diameter) its operational range moves within a fairly narrow interval, since the parameters can be altered only by changing the air velocity and adjustment of the blade angles.
Certain types of centrifuges are also used as sizers (see for example West-German Pat. No. 2 649 382). In these rotary drum type apparatuses the material to be separated flows in a carrier medium in the direction of the drum axis, and the sepration takes place with the aid of the discharge force applied to the grains. The flow time of the medium is selected to be less than the falling time of the smallest grains from the top of the liquid layer to the wall of the drum. Thus removal of the unsettled part of the grains finer than the given size does not represent problem. However, removal of the settled coarse grains is already difficult.
The course grains can be removed intermittently, in this case, however, the apparatus has to be stopped. Further drawback of this solution is the low output and, upon increasing the layer thickness, the running of the machine becomes more and more unstable. Consequently the already settled layer may become agitated again.
Another possibility is the discharge of the coarse grain layer with a worm, resulting in a more stable run of the apparatus, at the same time, however, the settling will be disturbed.
From the fluid mechanics point of view the best solution is the discharge with a nozzle which, however, is the least safe solution, since nozzles, have the tendency to become clogged, which may result in a change of the flow, or even in its stopping in some cases.
Thus in these centrifugal apparatuses, similarly to the spiral or zig-zag sizers, the fine product contains very few grains over the size limit, while the coarse fraction contains a fair amount of fine grains.